Mysterious noises that have awakened residents of outlying islands and rattled their windows recently are believed to come from a People's Liberation Army exercise being conducted in waters south of Hong Kong.

The exercise has also affected control of air space in the area. A spokeswoman for the Civil Aviation Department confirmed that the PLA was carrying out operations in southern waters but refused to disclose further details.

The spokeswoman said a block of airspace to the south, outside Hong Kong's territorial waters, had been "reserved" for the activities from 6am to noon from September 3 to 6.

"The CAD received prior notification of the airspace reservation and will keep aircraft clear of the area," she said.

The Security Bureau said it was not in a position to comment on defence matters.

However, residents have reported regular noises like distant thunder every morning from 6am until noon.

"It comes every five or 10 minutes every morning. I thought it might be some blasting work but it came from a long way off," a resident in south Lantau said.

She said she noticed that the windows in her house were rattling due to the noise.

She called the police about the noise but was told they knew nothing about it.

Lee Kwai-chun, a district councillor for Cheung Chau, said she heard some thundering sounds yesterday morning but thought it was caused by a rubbish collection truck. "I have no idea if there is a military drill or not," she said.

A spokeswoman for the Marine Department said they knew nothing of a PLA operation in the south. The PLA Hong Kong Garrison said it did not have any exercises near Hong Kong.

This article appeared in the South China Morning Post print edition as PLA exercise waking up islanders